Number of extractions in determination of effective cation-exchange capacity

The number of successive extractions with 1 M KCI needed for adequateestimation of effective cation-exchange capacity was studied with four mineral soils. The effective CEC estimated as the sum of equivalents of exchangeable Ca, Mg, Na, H and Al extracted by four successive treatments ranged from 57 to 206 meq/kg soil. In three cultivated soils, 63—90 % of CEC was saturated by Ca and Mg, in the fourth soil (a deeper layer virgin soil), 60 % of CEC by exchangeable H and Al. By two successive treatments of ten minutes duration with 50 ml of 1 M KCI, the equivalent sum of exchangeable cations extracted amounted to 83—92 % of effective CEC in cultivated soils and 67 % of that in virgin soil; >9O % of exchangeable Ca and Mg, 78—97 % of Al, 48—62 % of H and 28—64 % of Na were extracted. By three successive treatments the equivalent sum amounted to 79—96 % of effective CEC, by the single treatment of 30 minutes duration with 100 ml of 1 M KCI to 57—79 %. Two successive extractions with 1 M KCI may be enough for estimation of effective CEC in cultivated mineral soils with high degree of saturation by exchangeable Ca and Mg. Soils with high degree of saturation by exchangeable acidity require three successive extractions. Index words; effective cation-exchange capacity, exchangeable Ca, Mg, Na, H and A1


Introduction
In determination of exchangeable cations and effective cation-exchange capacity of soil, exchangeable cations are commonly extracted by successive treatments with unbuffered neutral salt solutions like 1 M KCI (e.g.Yuan 1959, Kaila 1971a, 1971 b, 1971 c, Niskanen and Jaakkola 1986).The normal practice, four or five successive extractions, presup- poses a laborious multiphase shaking and centrifugation procedure.A routine method with fewer successive extractions would facilitate analytical work.The purpose of this study was to investigate the possibility of reducing the number of successive extractions with 1 M KCI needed for adequate estimation of effective CEC in mineral soils.

Material and methods
The material consisted of four mineral soil samples from the Viikki Experimental Farm, University of Helsinki: three (No.2-4) from plough layer (0-20 cm) of cultivated soils and one (No. 1) from deeper layer (20-40 cm) of virgin soil (Table 1).The samples were air- dried and ground to pass a 2-mm sieve.The particle-size distribution of the inorganic mat- ter of soil was determined by the pipette meth- od (Elonen 1971).Soil pH was measured in soil-0.01M CaCl 2 suspension (1:2.5 v/v) (Ryti 1965).The organic carbon content was determined by a modified (Graham 1948)  Alten's wet combustion method.
Exchangeable cations were extracted from 10 g of soil by four successive treatments with 50 ml of 1 M KCI, shaking time 10 minutes.Uncombined extracts were analyzed for exchangeable Ca, Mg, Na, H and Al.Exchange- able cations were extracted from 10 g of soil  2. Exchangeable cations, meq/kg soil (% of total), extracted with 1 M KCI by four successive treatments and by a single treatment.

Results and discussion
The effective cation-exchange capacity esti- mated as the sum of exchangeable cations ex- tracted by four successive treatments of 1 M KCI was lowest in soil No. 1 with low pH and clay content and highest in soil No. 4 with the highest clay content (Table 2).The exchangeable cation composition of the virgin soil No.
1 included more H and A 1 than the other soils; 60 % of the estimated effective CEC of soil No. 1 was saturated with these acid cations (Table 3).The proportion of acid cations was lowest in soil No. 2 the pH of which was 5.3.
No exchangeable A 1 was found in this soil.This was in agreement with the observation of Kai- la (1971 a) that unbuffered 1 M KCI replaces A 1 only in samples with a pH(CaCl 2 ) below 5.3.In the other two soils with pH 4.5 and 4.6 the proportion of H and A 1 was about 10 % (Table 3).
The degree of saturation by exchangeable Ca and Mg in soils No. 2 and 4 was of the same order as in the material of Kaila (1972): 60-80 % and 10-30 % for Ca and Mg, respectively (Table 3).The degree of saturation by exchangeable Ca in soils No. 1 of corresponding soil groups in the material of Kaila (1972).The content of exchangeable Na in experimental soils (Table 2) of old sea bottom was much above average values reported by Kaila (1972).In soils 1 and 3 the proportion of Na was considerable, Na being the dominant basic cation in soil 1 (Table 3).The first 10-minute treatment with 1 M KCI extracted most of the total exchangeable Ca and Mg (Table 2), about 80 °7o or more was extracted in soils 2-4.The percentage was lower, <7O %, in soil 1 with low content of exchange- able Ca and Mg.As compared with the first 10minute treatment, the single 30-minute treat- ment with double quantity of extractant ex- tracted a little more Ca from soils I -3 1 -3 and less Ca from clay soil 4, while about equal amounts of exchangeable Mg were extracted by both treatments.
The second 10-minute treatment extracted 6-23 % of exchangeable Ca and 4-30 % of exchangeable Mg (Table 2).Thus, the first two successive 10-minute treatments extracted totally > 90 % of exchangeable Ca and Mg of the experimental soils.Compared to the single treatment with 100 ml of 1 M KCI the first two successive treatments with two 50 ml portions of 1 M KCI extracted 6-16 % and 5-30 °7o more of total exchangeable Ca and Mg, respectively, than the single treatment.
The extraction pattern of trivalent A 1 was rather similar to that of divalent Ca and Mg.The first treatment extracted nearly all the ex- changeable A 1 from soil 3 with low content of exchangeable A 1 and about 60 °7o from soils 1 and 4 (Table 2).The first two successive treatments with 50 ml of 1 M KCI extracted about 80 % of the exchangeable A 1 of these soils; the same amount was extracted by single treatment with 100 ml of 1 M KCI (Table 2).
The extraction pattern of monovalent ca- tions deviated from that of di-and trivalent cations.Most of Ca, Mg and A 1 was extracted by the first two treatments, while considerable quantities of exchangeable Na and H were ex- tracted by further treatments.The first treatment extracted only 20 % of the exchangeable Na in soil 1 and 38-47 % of that in the other soils (Table 2).The single treatment with 100 ml of KCI extracted slightly more Na from soils 2-4.An appreciable proportion (27 - 64 %) of exchangeable Na was still extracted by the fourth treatment.Extraction of exchangeable H continued evenly throughout the four successive treatments (Table 2).The single treat- ment with 100ml of 1 M KCI extracted more H than the first treatment with 50 ml of 1 M KCI.In soils 2 and 4 with high degree of satura- tion by exchangeable Ca and Mg, the equivalent sum of cations extracted by the first two successive treatments amounted to 92 °7o of effective CEC (Table 2), in soils 1 and 3 with high proportion of Na to 67 and 83 °7o and by the single treatment to >7O % in soils 2-4 (Table 2).By three successive treatments the equivalent sum amounted to >9O % in soil 3 and to about 80 % in soil 1.
In Finnish cultivated mineral soils the effec- tive CEC is overwhelmingly saturated with ex- changeable Ca and Mg (Kaila 1972, Niskanen andJaakkola 1986).Thus two treatments with 1 M KCI may be enough for estimation of effective CEC.Virgin soils with high degree of saturation by exchangeable acidity require more treatments.An adequate result can be achieved with three successive treatments as recommended by Bower et al. (1952) for ex- traction with neutral ammonium acetate.

Table 3 .
Exchangeable cations as % of effective CEC.